The present invention relates to an optical head apparatus suitable for use in an optical disc unit.
In an optical data recording/reproducing apparatus, in which a predetermined recording medium (optical disc) is irradiated with a light beam to record or reproduce data, it is necessary to focus the light on the disc, for a light spot to trace a track on the disc and to move the light spot toward the inner and outer circumferences. Therefore, the optical head apparatus is arranged so that it can achieve such focus control, tracking control and (radial) carriage control.
FIG. 2 illustrates the construction of a conventional optical head apparatus, which is applied to the compact disc player or optical video disc player or the like. As illustrated, an objective lens 2 is mounted on the upper surface of a substantially cylindrical armature 1. A post 3 is fixed upright on a carriage 11 to support the armature 1 so that the latter may vertically move or slide, and rotate. A focusing coil 4 is wound about the outer peripheral wall of the armature 1, and tracking coils 5 are stuck onto the focusing coil 4. Yokes 7 are fixed to the carriage 11 and permanent magnets 6 are attached thereto. Additional yokes 8 are fixed to the carriage 11. The yokes 8 and magnets 6 are respectively disposed inside and outside of the armature in such a way that they may oppose each other with the coils 4 and 5 interposed therebetween. An optical system 9 emits a laser beam 10 and is mounted on the carriage 11. Slide portions 12 are formed at the end portions of the carriage 11 and guide shafts 13 are inserted therethrough. An access (carriage) motor 14 rotates a screw 15. The screw 15 engages with a receiver portion 17 fixed to the carriage 11. A mirror 16 is fixed to the carriage 11.
The laser beam 10 emitted from a light source (not shown) such as a semiconductor laser or the like, which is incorporated into the optical system 9, is reflected by the mirror 16 to be directed into the objective lens 2, which in turn converges the incident light for irradiation onto a disc (not shown). The light reflected by the disc returns along the same path to be directed to the optical system 9. The optical system 9 incorporates photodiodes (not shown) to detect the light reflected from the disc.
A focus error signal is generated from the outputs of the photodiodes and is supplied to the coil 4. Since the coil 4 is disposed within the magnetic field of the magnets 6, when a current corresponding to the focus error signal flows, an electromagnetic force will be generated. As a result, guided by the post 3, the armature 1 (hence the objective lens 2) moves in the vertical direction (i.e., focusing direction). In this way, the focusing control is achieved.
Meanwhile, the tracking error signal generated from the outputs of the photodiodes is supplied to the coils 5. Since the coils 5 are also disposed within the magnetic field of the magnets 6, when the current flows, an electromagnetic force is generated. This electromagnetic force causes the armature 1 to be rotated in the clockwise or counterclockwise direction, with the post as a fulcrum. As a result, the tracking control is achieved.
Furthermore, when the data recording/reproducing position is moved in the direction of its inner or outer circumference, a carriage (radial) error signal is input to the access motor 14. At this time, the screw 15 is rotated by the motor 14. Since the receiver portion 17 engages the screw 15, guided by the shaft 13, the carriage 11 is moved radially of the disc with the result that carriage (radial) control is achieved.
As seen above, the optical head unit of FIG. 2, in which not only the optical system 9 but also the magnetic circuit and the like are loaded on the carriage 11, weighs as much as 50 g, and the average access speed with respect to a predetermined track is on the order of 300 ms, which is slow.
FIG. 3 shows another conventional optical head unit, in which the above-mentioned drawbacks are eliminated. As illustrated, a carriage 21 has slide portions 22 on its opposite end portions, and is supported by a pair of guide shafts 23 extending in the direction of the optical disc, so that the carriage can move along the guide shafts 23.
An armature 24 is mounted to the carriage 21, and an objective lens 25 is attached to the armature 24. A slide shaft 27 is fixed to the carriage 21 and is inserted through a slide portion 26 fixed to the armature 24. Coils 28 are for tracking and radial servo control of the carriage. Focusing coils 29 are stuck on the coils 28.
Yokes 31 are inserted through the respective coils 28, and permanent magnets 30 are disposed to generate magnetic fields extending to the yokes 31. Closed magnetic circuits are formed by the permanent magnets 30 and the yokes 31.
A laser beam 33 is emitted from an optical system 32, passed through a light transparent portion 34 formed at the slide portion 26 and the slide shaft 27, reflected at a mirror 35 disposed in the slide portion 27 and fixed onto the carriage 21, directed to the objective lens 25 and focused and irradiated onto the optical disc. Light reflected from the disc follows the same path in the opposite direction to be incident onto the optical system 32.
In the same way as the case described above, when the focus error signal is supplied to the coils 29, since the coils 29 are disposed within the magnetic field between the magnets 30 and the yokes 31, an electromagnetic force is generated with the result that the armature 24, with the objective lens 25, is moved in the vertical or focusing direction. At this time, the slide portion 26 is guided by the slide shaft 27 so that the armature 24 smoothly moves in the vertical direction. The height of the coils 28 are selected to be sufficiently higher than that of the yoke 31, so as to permit the vertical movement of the armature 24 required for focusing.
The coils 28 are driven in response to the tracking error signal and the carriage error signal. Since the coils 28 are also disposed within the same magnetic field generated by the permanent magnets 30, an electromagnetic force is generated so that the armature 24, with the objective lens 25 and carriage 21, moves along the guide shafts 23, and hence in the tracking and carriage-servo direction.
Since the optical system 32 in the apparatus of FIG. 3 is not loaded on the carriage 21 the entire carriage 21 can be made to weigh about 10 g. As a result, the average access time can be shortened to about 70 ms.
However, in the apparatus of FIG. 3, since the carriage 21 is moved not only for carriage control, which is a rough access conducted for the seek operation, but also for tracking control, which is a fine access on the order of microns, the load imposed during tracking is greater than in the apparatus of FIG. 2. Further, the friction on the slide portion 22 when it is moved produces a hysteresis phenomenon. Moreover, the mass distribution of the carriage 21 can cause an imbalance in the driving force of the coils 28, and the frequency characteristic changes depending on the amount of its movement. Consequently, with specific reference to the frequency characteristic, gain can suddenly be changed or the phase can be disturbed in the high range of bandwidth, thus making it difficult to achieve a precise tracking servo action.